The present invention relates to electrically heated windows, and more particularly to electrically heated windshields for aircraft.
Windshields for commercial aircraft are generally laminated structures formed of several layers of tempered glass and polymeric binding materials. Typically, a laminated aircraft windshield includes an outer glass face ply that is electrically heatable to prevent the accumulation of ice thereon. The outer glass face ply is laminated to a thicker, inner glass ply referred to as a main structural ply or pressure pane. Sometimes an additional structural ply, known as a fail-safe pane, is provided. The glass plies are typically bonded together with a clear polymeric sheet known as an interlayer. In addition to binding the glass plies together, the interlayer provides a flexible membrane to contain internal cabin pressure in the unlikely event of a complete glass pane failure.
In one type of heated aircraft windshield known in the industry, an electrical heating element consists of a thin layer of a transparent electroconductive material fused to the inner surface of the glass face ply. The face ply is bonded to a main structural glass ply by a polymeric interlayer as described above. The layer of electroconductive material is thus fused to the glass face ply on one side and adhered to the interlayer on the opposite side. The glass face ply is heated by passing an electric current across the windshield through the electroconductive layer. Electrical contact with the electroconductive layer is typically made with a pair of bus bars inserted between the glass face ply and the interlayer along opposite sides of the windshield. One advantage of such a windshield is that heat may be generated as a result of the electrical resistance of the electroconductive coating to maintain the temperature of the outer surface sufficiently high to prevent formation of ice or frost. Also, the absence of wire or other types of opaque heating elements embedded in the windshield improves the optical quality of the windshield.
Despite the advantages of the above-described windshield, certain disadvantages have been recognized. For example, the wide temperature range to which aircraft windshields are exposed during normal flight operations results in differential expansion of the polymeric interlayer and the glass plies. The interlayer has a higher coefficient of thermal expansion than that of either the glass face ply or the glass structural ply. Hence, the interlayer tends to expand and contract to a greater extent than the glass plies during temperature fluctuations, thereby resulting in substantial shear stresses being imposed on the bond between the glass plies. It has been observed that such shear stresses imposed by temperature fluctuations are particularly pronounced at the margins of the windshield. Additionally, various orthogonal stresses are imposed on the glass plies and the interlayer by the pressure differential between the pressurized aircraft cabin and the reduced pressure of the outside atmosphere. The various stresses imposed by thermal fluctuations and pressure differentials lead to three principal modes of failure of such windshields. First, bond failure, or delamination, occasionally occurs btween the glass plies and the interlayer. This results in deterioration of the optical quality of the windshield and also occasionally results in disruption and failure of portions of the electroconductive heating element.
Secondly, a phenomenon known in the industry as "cold chipping" is occasionally observed along the margins of the windshield where shear stresses are most pronounced. Cold chipping occurs when the interlayer thermally contracts due to cold temperatures and pulls chips of glass inwardly away from the edges of the outer glass face ply. Occasionally, these chips are deep enough to penetrate that portion of the glass that is subject to locked-in tension stresses caused by tempering, resulting in complete fracturing of the face ply.
Thirdly, a condition also due to the thermal contraction and stiffening of the interlayer is known as interlayer or vinyl cracking. This occurs when the resulting high stresses are not relieved by delamination or cold chipping, and is essentially a tensile or cohesive failure of the interlayer. This results in a loss of the ability to carry fail-safe pressure loads in the event of glass failure.
It is also recognized that the deterioration of aircraft windshields in the manner described above may be aggravated and accelerated by exposure of the polymeric resin constituting the interlayer to ultraviolet radiation and moisture, particularly around the edges of the windshield. Such exposure may cause the interlayer resin to deteriorate and become relatively hard and brittle. The brittle resin is thus less plastic and causes greater stresses to be applied to the bond line, the glass plies and the interlayer itself upon thermal contraction and expansion of the resin.
Yet another problem with conventional electrically heated windshields is the occasional fracturing of the outer face ply by hail. This occurs because the face ply must be kept sufficiently thin so that the electroconductive heating element may be positioned sufficiently close to the outer exposed surface to provide effective anti-icing capabilities. The strength of a glass ply may be increased by a thermal or chemical process known as tempering. The increase in strength that is obtainable is proportional to the thickness of the ply and has a marked effect on the fracture characteristics and residual visibility through the windshield in the event of impact damage. To ensure sufficient residual visibility, face plies are usually only partially tempered. As a result, and also because of the reduced thickness demanded by thermal requirements, face plies are not as strong as would be desired to effectively withstand all hail conditions.
Accordingly, it is a general object of the present invention to provide an improved electrically heated window suitable for use as an aircraft windshield. Specifically, it is an object of the present invention to provide an electrically heated aircraft windshield having a heatable main glass ply of unitary construction. It is also an object of the present invention to reduce the use of interlayers in electrically heated aircraft windshields. Finally, it is yet another object of the present invention to provide a heatable aircraft windshield that can be fabricated with a greater degree of strength for increased impact and abrasion resistance and improved durability.